1. Field of the Invention
The present invention generally relates to a circuit protective device with a positive temperature coefficient (hereinafter "PTC") element and also to an electric junction box with the circuit protective device and, more particularly, to the circuit protective device and to the electric junction box wherein a low resistance conductive passage made of a carbonaceous powder to be released from the PTC element is not formed between the electrodes of the PTC element.
2. Description of the Related Art
In a circuit protective device for overload current using a PTC element made of a conductive polymeric composition having PTC characteristics, an electric circuit is shut off by raising the electric resistance of the PTC element due to temperature rise thereof caused by an abnormal current.
Such a circuit protective device is provided in an electric junction box mounted on a vehicle such as an automobile, while keeping a safe state when the circuit is shut off by the PTC element. More specifically, since the PTC element is apt to release carbonaceous powder and the powder would accumulate on a circuit board on which the PTC element is attached through lead wires thereof, a low resistance conductive passage could be formed, thereby causing a failure of the circuit.
In Japanese Patent Application Laid-open No. 58-190230, a circuit protective device with a structure described below for keeping the above safe state is disclosed.
Referring to FIG. 3, the circuit protective device 1 is constructed such that a cup-like electrode 4 having a lead wire 3 is attached to each of the ends of a PTC element 2, the PTC element 2 along with the electrodes 4 is accommodated in a metal tube 6 covered with an insulating tape 5, and insulating circular plates 7 are attached to the respective ends of the tube 6. The circuit protective device 1 is mounted on a circuit board (not shown) as a parts of an electric circuit shown in FIG. 4. Here, reference numerals 8 and 9 indicate a power unit and an electrical load, respectively.
In this prior art, forming of the above low resistance conductive passage is prevented by means of accommodating the PTC element 2 in the metal tube 6 with the insulating circular plates 7, thereby keeping the safe state.
With respect to the prior art circuit protective device, however, a number of parts required for the device causes the high cost and simultaneously complicated assembly work. This, of course, causes an increase in the cost of an electric junction box with the circuit protective device 1.